The invention relates to a process for preparing hydrocyanic acid (HCN) by catalytic dehydration of gaseous formamide in the presence of oxygen.
Hydrocyanic acid is an important basic chemical which is used as starting material in numerous organic syntheses such as the preparation of methacrylic esters, lactic acid and metal cyanides, as polyamide precursor, for preparing pharmaceutical and agrochemical products, in mining and in the metallurgical industry.
An important process for the industrial preparation of hydrocyanic acid is thermal dehydration of formamide under reduced pressure, which proceeds according to the equation (I):HCONH2→HCN+H2O  (I)
This reaction is accompanied by the decomposition of formamide according to equation (II) to form ammonia and carbon monoxide:HCONH2→NH3+CO  (II)
The ammonia formed catalyzes the polymerization of the desired hydrocyanic acid and thus leads to a deterioration in the quality of the hydrocyanic acid and a reduction in the yield of the desired hydrocyanic acid.
Soot is formed in the preparation of hydrocyanic acid and its formation can, as disclosed in EP-A 0 209 039, be suppressed by the addition of small amounts of oxygen in the form of air. In the disclosure of EP-A 0 209 039, a process for the thermolytic dissociation of formamide over densely sintered shaped aluminum oxide or aluminum oxide-silicon dioxide bodies or over shaped chromium-nickel stainless steel bodies which are corrosion-resistant at high temperature is described. Since the conversion of formamide in this process is incomplete, DE-A 101 38 553 suggests recirculating unreacted residual formamide to the formamide dissociation.